seeing that you are a magus wizard i would highly suggest that you complete not just mooshu but also dragonspyre before you even attempt wintertusk. Wintertusk is about the difficulty of celestia(more or less). I hope this helped. :)
you wont be in the 4th gh questline until 5 more levels. (its not as easy as the other parts either so i suggest doing it once you get your level 48 spell.) after finishing that you'll probably be almost level 50. i suggest being at least level 54 in wt because you need a lot of critical and block. in other words, do ms and most of ds (people usually become level 48 around labyrinth.) then do gh's 4th installment. start on cl and about half way through get your lvl 54 gear and start wt. another thing: dont be fooled by the wt boosses' lack of health. they are vicious and i had to learn the hard way
Mooshu, as there are areas of Wintertusk that are designed to be completed when you are closer to Legendary (level 60); plus it will put you that much closer to finishing the first storyline arc and GH/WT is side world that can be done whenever, especially when you need a break from the storyline.